Method for Determining a Soot Load of a Particle Filter Provided with a Selective Catalytic Coating

ABSTRACT

A method for determining a soot load on a particle filter provided with a selective catalytic coating is disclosed. The method includes determining a nitric oxide conversion on the particle filter and determining a soot load on the particle filter from the determined nitric oxide conversion.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention concerns a method for determining a soot load of aparticle filter with a selective catalytic coating.

Current and future emission guidelines stipulate clear limits onemissions of internal combustion engines, mainly with respect tohydrocarbon, carbon monoxide, nitric oxide and particle emissions. Theincreasing fuel consumption savings in the operation of internalcombustion engines mean falling exhaust gas temperatures available forthe catalytic exhaust aftertreatment. For this reason, close-coupled SCR(selective catalytic reduction) systems, having a close-coupled particlefilter with integrated selective catalytic coating and SCR coating(SDPF), are playing a growing role in future exhaust aftertreatmentdesigns in order to meet the increased requirements. For such an exhaustaftertreatment system, it is important to be able to determine the sootload of the particle filter as accurately as possible. Overly frequenttriggering of a regeneration results in greater aging of the catalyticcoating applied on the filter as well as higher carbon dioxide andpollutant emissions of the internal combustion engine. On the otherhand, if the regeneration is initiated too late the result can beexcessively high temperature gradients in the particle filter that cancause mechanical damage.

A method is known from European patent application EP 2 749 745 A1 fordetermining a patent application of a particle filter with a selectivecatalytic coating in which two different determination methods arecombined with each other. For one, a soot raw emission model or loadmodel is used that calculates a soot emission depending on variousengine parameters and determines the quantity deposited in the particlefilter and removed by the continuous soot combustion with nitrogendioxide (NO2). Another model is used that calculates the soot load bymeasuring a differential pressure in the exhaust gas flow fallingthrough the particle filter (differential pressure model).

A disadvantage of this is that for safety reasons—in particularcomponent protection reasons—the regeneration of the particle filter isalways triggered if one of the two models reaches the maximum allowableload limit stored in the internal combustion engine's control device. Itis found that the determination of the soot load from the differentialpressure is very imprecise, especially after an incomplete regeneration,also called partial regeneration. The soot raw emission model, for itspart, has very sharp fluctuations in precision depending on the internalcombustion engine's operating point, so there can be sharp fluctuationsbetween the soot load values for the particle filters calculated bymeans of the two models. The overall result is a possible prematureregeneration of the particle filter without it actually being necessaryat the time indicated by one of the models.

The invention is based on the object of creating a method that does nothave these disadvantages.

In particular, the object is solved by creating a method for determininga soot load of a particle filter having a selective catalytic coating,namely a particle filter with a coating having a selective catalyticeffect for reduction of nitrogen oxides, with the following steps: anitric oxide conversion—especially current—is determined on the particlefilter and a soot load of the particle filter—especially current—isdetermined from the particular nitric oxide conversion. In particular,the soot load of the particle filter is determined depending on theparticular nitric oxide conversion. The nitric oxide conversion can alsobe determined by averaging individual nitric oxide conversion valuesdetermined in a predefinable time interval ranging from a few seconds toa few minutes. It has been shown that with a particle filter having aselective catalytic coating, interactions are created between thefilter's soot load and the nitric oxide conversion in the filter walldue to the spatial merging of the selective catalytic reduction functionon the one hand and the particle filter function on the other. Thisdependence makes it possible to deduce the particle filter's soot loadfrom the nitric oxide conversion. In at least certain operating ranges,this method has a very high accuracy and is therefore suitable fordetermining a highly precise soot load that can then be used as thebasis for deciding on a regeneration measure without having to fear apremature regeneration. In particular, this can also be combined with atleast one other determination of the soot load based on at least oneother model, wherein the very precise method addressed here inparticular can be used to correct a soot load determined based on theother model correct or adapt the other model itself.

The method is preferably performed in an exhaust tract of an internalcombustion engine, wherein the exhaust tract preferably has an oxidizingcatalytic converter, in particular a diesel oxidation catalyst (DOC),and/or a nitric oxide storage catalyst (NSC). In the flow direction ofthe exhaust gas downstream of the oxidizing catalytic converter and/orthe diesel oxidation catalyst is a downstream system for selectivecatalytic reduction of nitric oxides, which has the particle filter thathas the selective catalytic coating. With one embodiment of the methodit is possible that this is done for an SCR system that, in addition tothe particle filter provided with the selective catalytic coating, has acatalytic converter (SCR catalytic converter) without particle filterfunction designed for selective catalytic reduction. This is thenpreferably arranged downstream of the particle filter provided with theselective catalytic coating.

Preferably, an exhaust gas recirculation device is also used.Particularly preferred is a multipath exhaust gas recirculation devicehaving a high-pressure exhaust gas recirculation line and a low-pressureexhaust gas recirculation line.

In a preferred embodiment of the method the soot load is determined fromthe predetermined nitric oxide conversion by the specific nitric oxideconversion being compared with a nitric oxide conversion—preferablystored in a characterization field—having the particle filter providedwith the selective catalytic compound when it is not loaded with soot;for example, when new or after a complete regeneration. In particular,the current soot load can be determined very precisely by the comparisonwith this reference value of the particle filter without soot load.

An embodiment of the method is preferred that is characterized in thatthe soot load is determined with the procedure described here if anexhaust gas temperature—in particular current—that is preferablymeasured by a temperature sensor arranged in the exhaust tract upstreamof the particle filter—is greater than or equal to a predeterminedminimum temperature and less than or equal to a predetermined maximumtemperature. It has been shown that there is a clear and, in particular,clearly evaluable connection between the nitric oxide conversion and thesoot load of the particle filter, especially in a particular temperaturerange for the exhaust gas temperature upstream of the particle filter.Therefore, the method can be carried out with a particularly highaccuracy in this temperature range. It is seen that the minimumtemperature preferably is at least 175° C. to at most 210° C.Alternatively or additionally, the maximum temperature is preferablyfrom at least 240° C. to at most 280° C. In a preferred embodiment ofthe method the soot load is preferably determined when the exhaust gastemperature upstream of the particle filter is from at least 150° C. toat most 300° C., preferably from at least 175° C. to at most 280° C.,preferably from at least 175° C. to at most 240° C., preferably from atleast 210° C. to at most 280° C., preferably from at least 210° C. to atmost 240° C. The temperature ranges stated here are particularlyappropriate areas for a highly accurate determination of the soot loadbased on the determined nitric oxide conversion. The minimum temperatureand/or maximum temperature can preferably be variably set in acontroller of an internal combustion engine.

An embodiment of the method is also preferred that is characterized inthat for determining the soot load from the specific nitric oxideconversion a—particularly current—ratio of a nitrogen dioxideconcentration to a total nitric oxide concentration in the exhaust gasis used upstream of the particle filter that is the sum of the nitrogendioxide concentration and a nitrogen monoxide (NO) concentration. It hasbeen shown that the dependence of the nitric oxide conversion on thesoot load itself additionally depends on the ratio of the nitrogendioxide concentration to the total nitrogen oxide concentration in theexhaust gas. In particular, the nitric oxide conversion rises stronglywith an increasing ratio of nitrogen dioxide to total nitrogen oxidewith rising soot load. A very precise evaluation of the soot load basedon the nitric oxide conversion can therefore be performed if the ratioof nitrogen dioxide to total nitrogen oxide in the exhaust gas isconsidered.

An embodiment of the method is also preferred that is characterized inthat the soot load is determined if the—in particular current—ratio ofthe nitrogen dioxide concentration to the total nitrogen oxideconcentration in the exhaust gas upstream of the particle filter isgreater than a predetermined minimum value. This can be defined suchthat at any rate a very accurate determination of the soot load ispossible based on the nitric oxide conversion. Since the sensitivity ofthe method rises with the rising ratio of nitrogen dioxide to totalnitrogen oxide, a very high accuracy can be assured through a suitabledefinition of the predetermined minimum value. An embodiment of themethod is preferred in which the predetermined minimum value is from atleast 10% to at most 50%, preferably from at least 20% to at most 40%,and particularly preferably from at least 30% to at most 50%. In theseranges, a highly accurate evaluation of the soot load depending on thepredetermined nitrogen oxygen conversion is guaranteed.

An embodiment of the method is preferred that is characterized in thatthe nitrogen oxygen conversion at the particle filter is determined bymeans of nitric oxide sensors arranged upstream and downstream of theparticle filter. Such sensors are preferably provided in the exhausttract anyway, so that no additional, expensive elements are needed forperforming the method. The method can therefore be performed verycost-effectively. At the same time, the nitric oxide conversion at theparticle filter can be determined very accurately with the aid ofsensors provided upstream and downstream of it. In this case, adifferential measurement of the total nitrogen oxide concentration inthe exhaust gas between the measuring point upstream and the measuringpoint downstream of the particle filter is preferably carried out. Thenitric oxide conversion can thus be determined very simply and veryprecisely at the same time.

A nitric oxide sensor arranged downstream of the particle filter canalso be arranged downstream of an additional SCR catalytic converterwithout particle filter function provided downstream of the particlefilter.

An embodiment of the method is also preferred that is characterized inthat the ratio of the nitrogen dioxide concentration to the totalnitrogen oxide concentration in the exhaust gas is determined upstreamof the particle filter based on the—in particular current—exhaust gastemperature upstream of the particle filter or upstream of a—inparticular arranged upstream of the particle filter—oxidation catalyst,an exhaust gas mass flow over the oxidation catalyst, and/or an agingstate of the oxidation catalyst. Especially preferably, the ratio ofnitrogen dioxide to total nitrogen oxide is read depending on at leastone of the named parameters from the characterization field. This isparticularly preferably stored in a control device of the internalcombustion engine that has the exhaust tract in which the method isperformed. Each of the parameters named here by itself characterizes themethod addressed here, where in particular the named parameters togetherenable a very precise determination of the ratio so that they are suitedto spanning a characteristic field for the ratio.

An embodiment of the method is also preferred that is characterized inthat the soot load is compared by comparison of the determined nitricoxide conversion with a predetermined nitric oxide conversion of theunloaded particle filter; i.e., without soot load. The method thusperformed achieves a particularly high accuracy, as was alreadyexplained in greater detail above. The predetermined nitric oxideconversion of the unloaded particle filter is preferably determineddepending on an exhaust gas mass flow over the particle filter, theexhaust gas temperature upstream of the particle filter, the ratio ofnitrogen dioxide to total nitrogen oxide upstream of the particlefilter, a reducing agent load, in particular an ammonium (NH3) load, anaging status of the particle filter and/or an operating point of aninternal combustion engine, the exhaust tract of which has the particlefilter. Particularly preferably, the predetermined nitric oxideconversion is read depending on at least one of the parameters here froma characterization field stored in particular in a control device forthe internal combustion engine. Each of the parameters here by itself isalready characteristic for the predetermined nitric oxide conversion,but in particular, the parameters named here in combination with eachother are characteristic for the predetermined nitric oxide conversion,so that they are particularly suited to spanning a characterizationfield for the predetermined nitric oxide conversion. In particular,preferably the nitric oxide conversion of the particle filter not loadedwith soot depending on the exhaust gas mass flow over the particlefilter, the temperature upstream of the particle filter, the nitrogendioxide to nitric oxide ratio before the particle filter, the reducingagent load of the particle filter, and the aging status of the particlefilter are stored in the control device and read based on these factorsfor the current operating state of the internal combustion engine. Thereducing agent load of the particle filter is preferably determined withthe aid of a suitable model, in particular calculated.

At least one of the aging statuses of the oxidation catalyst and/or theparticle filter is preferably determined as aging factor, and inparticular dependent on the factors of temperature over time. Inparticular, a temperature-time integral can be formed for at least oneof the named exhaust aftertreatment elements from which an aging statusis then determined.

An embodiment of the method is also preferred that is characterized inthat the soot load of the particle filter, in addition to the procedurepreviously described, is determined in the method by means of a loadmodel and also additionally based on a differential pressure fallingover the particle filter, in particular through a differential pressuremodel. There are then three options available for determining the sootload, which can be used alternatingly for correction of the determinedsoot load and thus in combination with each other and assure aparticularly high accuracy of the determination of the soot load. Aregeneration of the particle filter is preferably performed if one ofthe determination methods returns a soot load as the result that reachesor exceeds a predetermined maximum value for the soot load. This canguarantee that the particle filter is always regenerated if this isindicated.

An embodiment of the method is also preferred that is characterized inthat the soot load is determined based on the nitric oxide conversion ifa first load value for the soot load determined based on the load modeland a second load value for the soot load determined based on thedifferential pressure have a difference that reaches or exceeds apredetermined difference threshold. The very precise method fordetermining the soot load through the determined nitric oxide conversiondescribed here is therefore preferably used if the two otherdetermination methods yield sharply differing results, where this is anindication that at least one of the determination methods currentlyyields no reliable load values. In a preferred embodiment of the method,the soot load determined based on the nitric oxide conversion ispreferably used for correction of the first and/or the second load valueand/or for correction of the determination methods on which these loadvalues are based. This is based on the idea that the soot loaddetermined using the method from the nitric oxide conversion—at least inparticular operating ranges—is the most accurate possible soot loaddeterminable. This can thus be used in a particularly favorable way forcorrection of the load values otherwise determined and/or thedetermination methods for determining these load values. It is possiblethat a regeneration of the particle filter is postponed even though thisis indicated by one of the models, if the method according to which thesoot load is determined based on the nitric oxide conversion determinedindicates no regeneration. In this case, the determination methodindicating a regeneration and/or the load value determined based on thisdetermination method is instead corrected with the aid of the load valuedetermined by the method described here.

Last of all, an embodiment of the method is preferred that ischaracterized in that conditions are deliberately set by control of anexhaust gas recirculation device and/or by active heating of anoxidation catalyst under which the soot load of the particle filter canbe determined from the determined nitric oxide conversion. An exhaustgas recirculation device is preferably used that has a high-pressureexhaust gas recirculation line and a low-pressure exhaust gasrecirculation line. A strategy for controlling the exhaust gasrecirculation device can then be adapted as needed such that the exhaustgas recirculation occurs completely through the high-pressure exhaustgas recirculation line, thus achieving the best possible nitrogendioxide formation on the oxidation catalyst. Furthermore, in addition oralternatively to a change of the total exhaust gas recirculation rateand/or by active heating of the oxidation catalyst, in particular bymeans of an electric heating device, a desired temperature range for theexhaust gas temperature can be specifically set. Overall, it is thuspossible to create conditions regarding the exhaust gas temperature orregarding the ratio of nitrogen dioxide to total nitrogen oxide in theexhaust gas in which a very accurate determination of the soot load ispossible with the aid of the method described here.

The deliberate setting of such favorable conditions is preferably donein particular if the two other determination methods, i.e., inparticular the load model and the differential pressure model, yieldsharply different load values, where a difference between the loadvalues preferably reaches or exceeds a predetermined differencethreshold.

The invention also includes a control device for an internal combustionengine, which is designed for performing a preferred embodiment of themethod. It is possible that the control device is the engine controlunit (ECU) of the internal combustion engine. Alternatively, it is alsopossible that a separate control device is provided for performing themethod.

It is possible that the method is implemented directly in an electronicstructure, in particular a hardware, of the control device.Alternatively or additionally, it is possible that certain proceduralsteps, preferably the entire method, are present as a computer programproduct. In this respect, preferably a computer program product isloaded into the control device that has steps based on which anembodiment of the method is performed if the computer program productruns on the control device.

The invention also includes an internal combustion engine, adapted toperform an embodiment of the method and/or having a control device thatis designed for performing the method. In particular, an exhaust tractwith at least one of the exhaust aftertreatment elements described inthe method is assigned to the internal combustion engine.

Finally, the invention includes also a motor vehicle having an internalcombustion engine according to any one of the embodiments describedabove. The motor vehicle is preferably configured as a passenger car,truck or commercial vehicle.

The invention is described in more detail below with reference to thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an internal combustion enginewith exhaust tract, for which an embodiment of the method fordetermining a soot load is feasible;

FIG. 2 is a schematic representation of the connection between thenitric oxide conversion in a particle filter provided with a selectivecatalytic coating and the soot load of same depending on thetemperature;

FIG. 3 is a schematic representation of the connection between thenitric oxide conversion and the soot load depending on a ratio of anitrogen dioxide concentration to a total nitrogen oxide concentrationin the exhaust gas upstream of the particle filter, and

FIG. 4 is a schematic representation of an embodiment of the method.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of an embodiment of an internalcombustion engine 1 with an exhaust tract 3, for which the methoddescribed here is preferably performed. The exhaust tract 3 comprises aparticle filter 5 provided with a selective catalytic coating arrangedto filter soot particles from the exhaust stream of the internalcombustion engine 1 and selectively catalytically reduce nitric oxidespresent in the exhaust gas. Such a particle filter 5 is generally alsocalled SDPF. Downstream of the particle filter 5 in the embodimentpresented here is additionally arranged an SCR catalytic converter 7,which is likewise arranged for selective catalytic reduction of nitricoxides but has no particle filter function. Upstream of the particlefilter 5 is arranged a dosing device 9 for a reducing agent, which canbe dosed into the exhaust stream by means of the dosing device 9, whereit is then converted with the nitric oxides on the selective catalyticcoating of the particle filter 5 and the SCR catalytic converter 7,whereby the nitric oxides are reduced to nitrogen.

As the reducing agent, a urea-water solution is preferably dosed throughthe dosing device 9 into the exhaust stream, wherein the urea reactswith the hot exhaust gas and decomposes with formation of ammonium,which then acts as the actual reducing agent in the particle filter 5and preferably in the SCR catalytic converter 7.

To be able to determine the nitric oxide conversion in the particlefilter 5, in the embodiment presented here upstream of the dosing device9 is arranged a first nitric oxide sensor 11, where directly downstreamof the particle filter 5—here in particular between the particle filter5 and the SCR catalytic converter 7—is arranged a second nitric oxidesensor 13. The second nitric oxide sensor 13 can also be arrangeddownstream of the SCR catalytic converter 7. A current absolute orrelative nitric oxide conversion in the particle filter 5 can bedetermined through a differential measurement of the signals of the twonitric oxide sensors 11, 13, in particular taking into consideration thesignal of the first nitric oxide sensor 11.

In the embodiment shown here, an oxidation catalyst 15 is arrangedupstream of the particle filter 5, and in particular also upstream ofthe dosing device 9. Instead of the oxidation catalyst 15, or inaddition to the oxidation catalyst 15, it is possible that upstream ofthe particle filter 5, and in particular upstream of the dosing device9, is arranged a nitric oxide storage catalyst (NSC).

The oxidation catalyst 15 has in this case a heating device 17 which ispreferably designed as an electrical heating device.

The internal combustion engine 1 also has a charge air line 19 throughwhich charge air can be fed to it. In an inherently known manner, acompressor 21 is arranged in the charge air line that can be driven by aturbine 23 arranged in the exhaust tract 3. To this extent, aturbocharger device is realized with the embodiment shown here.

The embodiment shown also has an exhaust gas recirculation device 25,which here has a high-pressure exhaust gas recirculation line 27 and alow-pressure exhaust gas recirculation line 29. The high-pressureexhaust gas recirculation line 27 upstream of the turbine 23 branchesoff from the high-pressure section of the exhaust tract 3, entering thehigh-pressure section of the charge air line 19 downstream of thecompressor 21. The low-pressure exhaust gas recirculation line 19branches downstream of the SCR catalytic converter 7 from thelow-pressure section of the exhaust tract 3 and enters the low-pressuresection of the charge air line 19 upstream of the compressor 21.

In the method, the heating device 17 is preferably deliberatelycontrolled to bring the exhaust gas temperature into a range in whichthe method can be performed with high accuracy. Alternatively oradditionally, the exhaust gas recirculation device 25 is also controlledin a manner creating conditions in which the method can be performedwith high accuracy. It is provided in particular that the exhaust gasrecirculation overall is shifted to the high-pressure exhaust gasrecirculation line 27 to achieve the highest possible nitrogen dioxideformation at the oxidation catalyst 15. Additionally or alternatively,it is possible to specifically influence an overall exhaust gasrecirculation rate to bring the exhaust gas temperature—additionally oralternatively to control of the heating device 17—into a range in whichthe method can be performed especially efficiently and with highaccuracy. For determination of the exhaust gas temperature, atemperature sensor not shown in FIG. 1 is also preferably provided withwhich the exhaust gas temperature can be detected upstream of theparticle filter 5.

FIG. 2 shows a schematic, diagrammatic illustration of a dependence ofthe nitric oxide conversion (NOx)—plotted against the exhaust gastemperature T upstream of particle filter 5—on the soot load of particlefilter 5. With a solid curve 31 is shown the nitric oxide conversion atparticle filter 5 with a first, higher soot load of particle filter 5,wherein with a second, dashed curve 35 the nitric oxide conversion onparticle filter 5 is plotted with a second, lower soot load against thetemperature of the exhaust gas before particle filter 5. It is alsosuggested in FIG. 2 that there is a temperature range between a minimumtemperature Tmin and a maximum temperature Tmax in which the method canbe applied with particularly high sensitivity, because curves thatdescribe the nitric oxide conversion U(NOx) dependent on the temperatureT for differing soot loads of particle filter 5 have comparatively largedistances from each other. In particular, there are also no curveintersections in this temperature range so that a clear assignment ofthe nitric oxide conversion U(NOx) to the soot load is possible. This isespecially true when—as is preferably carried out—the current nitricoxide conversion is compared with the reference value for the unloadedparticle filter 5 and to this extent with a baseline or base curve.

FIG. 3 shows schematically and diagrammatically the dependency of thenitric oxide conversion U(NOx)—plotted against the temperature T of theexhaust gas upstream of particle filter 5—on a ratio of a nitrogendioxide concentration to a total nitric oxide concentration in theexhaust gas. In the diagram of FIG. 5 a first pair of curves 35 is drawnshowing nitric oxide conversions with a first, higher ratio of nitrogendioxide to total nitric oxide. A first curve 37, presented solid, showsthe temperature dependency of the nitric oxide conversion for a first,higher soot load of particle filter 5, and a second, dot-dashed curve 39shows this progression for a second, lower soot load of particle filter5. A second pair of curves 39 is shown according to a second, lowerratio of nitrogen dioxide to total nitric oxide in the exhaust gas. Thissecond pair of curves 39 has a third, dashed curve 41 showing the nitricoxide conversion depending on the temperature for the first, larger sootload of particle filter 5, and the second pair of curves 39 has afourth, dotted curve 43 showing the nitric oxide conversion depending onthe temperature for the second, lower soot load of particle filter 5.The pairs of curves 35, 39 are based on identical first and second sootloads. It is immediately clear from FIG. 3 that there is a clearerseparation of the nitric oxide conversion depending on the soot load ofparticle filter 5 if the ratio of nitrogen dioxide to total nitric oxideis higher. The method can be performed with particularly high accuracygiven a comparatively high ratio of nitrogen oxide to total nitricoxide, especially if a predetermined minimum value for the ratio isexceeded. This is between at least 10% to at most 50%, preferablybetween at least 20% to at most 40%, and especially preferably betweenat least 30% to at most 50%.

FIG. 4 shows a schematic representation of an embodiment of the methodas a flowchart. In the method, the current nitric oxide conversion 45 atthe particle filter 5 is preferably determined with the aid of thenitric oxide sensors 11, 13. From the characteristic field 47 is used asreference value the load-free nitric oxide conversion 49 that theparticle filter 5 has if it is not loaded with soot, i.e., either new orcompletely regenerated. The characteristic field 47 is spanned over anaging factor 51 for particle filter 5, the ratio 53 of the nitrogendioxide concentration to the total nitric oxide concentration upstreamof particle filter 5 in the exhaust gas, the temperature 55 upstream ofparticle filter 5 in the exhaust gas, an exhaust gas mass flow 57 overparticle filter 5, and a reducing agent load 59, in particular anammonia load of particle filter 5.

The load-free nitric oxide conversion 49 is accordingly read dependingon the parameters shown on the left of characteristic field 47.Preferably, the aging factor 51 and the ratio 53 of nitrogen dioxideconcentration to total nitric acid concentration is dimensionless. Thetemperature 55 is preferably given in ° C., the exhaust gas mass flow 57preferably in kg/h, and the reducing agent loading 59 preferably in g.

The aging factor 51 is preferably determined depending on the factorstemperature and time, in particular as a temperature-time integral.

The ratio of nitrogen dioxide to total nitric oxide in the exhaust gasis preferably determined based on the temperature of the exhaust gasupstream of the oxidation catalyst 15 or the temperature upstream ofparticle filter 5, the exhaust gas mass flow over the oxidationcatalyst, and the aging status of the oxidation catalyst 15, likewisefrom a characterization field not shown here. The reducing agent load 59preferably results from a model calculation.

In a differential element 61, a difference 62 is now preferably formedbetween the current nitric oxide conversion 45 and the load-free nitricoxide conversion 49. This difference 62 goes into a detection member 63,in which the ratio 53 of the nitrogen dioxide concentration alsopreferably enters into the total nitric oxide concentration. From thisratio 53 and the difference 62 determined in the difference member 61,the detection member 63 now calculates the current soot load 65 ofparticle filter 5. This represents a very accurate value for the sootload of particle filter 5 in particular in the temperature range optimalfor the method, and with a ratio 53 exceeding the predetermined minimumvalue. In particular, it is possible to use this value for correction ofother determination methods, in particular a soot load model and/or adifferential pressure model.

Overall, it is shown that a premature regeneration of particle filter 5in particular can be avoided with the aid of the method. This extendsthe regeneration interval of particle filter 5, resulting in a savingsof fuel and a lower thermal load on the catalytic coating of the exhaustaftertreatment system.

1.-10. (canceled)
 11. A method for determining a soot load of a particlefilter provided with a selective catalytic coating, comprising the stepsof: determining a nitric oxide conversion on the particle filter; anddetermining a soot load of the particle filter from the determinednitric oxide conversion.
 12. The method according to claim 11, whereinthe soot load is determined if an exhaust gas temperature in an exhausttract upstream of the particle filter is greater than or equal to apredetermined minimum temperature and less than or equal to apredetermined maximum temperature and wherein the predetermined minimumtemperature is from at least 175° C. to at most 210° C. and/or thepredetermined maximum temperature is from at least 240° C. to at most280° C.
 13. The method according to claim 11, wherein the soot load isdetermined if an exhaust gas temperature in an exhaust tract upstream ofthe particle filter is from at least 150° C. to at most 300° C.
 14. Themethod according to claim 11, wherein for the determining the soot loadfrom the determined nitric oxide conversion, a ratio of a nitrogendioxide concentration to a total nitrogen oxide concentration in exhaustgas upstream of the particle filter is used.
 15. The method according toclaim 14, wherein the soot load is determined when the ratio of thenitrogen dioxide concentration to the total nitrogen oxide concentrationis greater than a predetermined minimum value and wherein thepredetermined minimum value is at least 10% to at most 50%.
 16. Themethod according to claim 11, wherein the nitric oxide conversion on theparticle filter is determined by respective nitric oxide sensorsdisposed upstream and downstream of the particle filter.
 17. The methodaccording to claim 14, wherein the ratio is determined based on atemperature of the exhaust gas upstream of the particle filter or atemperature of the exhaust gas upstream of an oxidation catalyst, a massflow of the exhaust gas over the oxidation catalyst, and/or an agingstatus of the oxidation catalyst.
 18. The method according to claim 11,further comprising the step of comparing the determined nitric oxideconversion with a predetermined nitric oxide conversion of an unloadedparticle filter, wherein the predetermined nitric oxide conversion isdetermined depending on an exhaust gas mass flow over the particlefilter, an exhaust gas temperature upstream of the particle filter, aratio of a nitrogen dioxide concentration to a total nitrogen oxideconcentration upstream of the particle filter, a reducing agent load ofthe particle filter, an aging status of the particle filter, and/or onan operating point of an internal combustion engine that has theparticle filter.
 19. The method according to claim 11, wherein the sootload of the particle filter is additionally determined by a load modeland a differential pressure model, wherein a regeneration is performedif one of the soot load determinations returns a soot load that reachesor exceeds a predetermined maximum value.
 20. The method according toclaim 19, wherein the soot load is determined from the determined nitricoxide conversion if a determined first load value based on the loadmodel and a determined second load value based on the differentialpressure model have a difference that reaches or exceeds a predetermineddifference threshold.
 21. The method according to claim 20, wherein thesoot load determined from the determined nitric oxide conversion is usedfor correction of the first and/or the second load values and/or forcorrection of at least one of the load model and the differentialpressure model.
 22. The method according to claim 11, wherein specificconditions are set by controlling an exhaust gas recirculation deviceand/or by active heating of an oxidation catalyst under which the sootload of the particle filter is determined from the determined nitricoxide conversion.